In a superconducting magnet device applied to an NMR apparatus, etc., a refrigerator is used for recondensing an evaporated coolant so as to reduce the consumption of liquid helium as a coolant to zero. However, the periodic liquefaction cycle of the refrigerator generates a pressure fluctuation in a coolant tank housing a coolant and brings about a slight displacement in the coolant tank. As a result, a disturbance occurs in the magnetic field of the measurement target to produce harmful noise in the measurement results.
To cope with this problem, Patent Document 1 discloses a cryostat assembly where a plug having provided therein a large number of holes each having a diameter sufficiently smaller than the wavelength of an acoustic wave generated by the refrigerator is provided in a passage connecting a recondensing chamber housing the lower part of the refrigerator to a coolant tank housing liquid helium. The helium gas produced by evaporation of liquid helium in the coolant tank enters the recondensing chamber through the large number of holes. The liquid helium produced by the recondensation of helium gas in the recondensing chamber returns to the coolant tank through the large number of holes. An acoustic wave generated in the refrigerator is attenuated during passing through the large number of holes.